1. Field of the Invention
The present invention relates to a four-stroke engine and an outboard motor provided with the engine.
2. Description of the Related Art
A direct-injection engine is known in which liquid fuel is supplied directly to a combustion chamber so as to improve its output and its fuel efficiency.
In the direct-injection engine, fuel is vaporized in a cylinder, and, as a result, the inside of the cylinder is cooled. Therefore, the output of the engine can be improved by increasing the amount of air with which the cylinder is filled. In addition, the temperature of the inside of the cylinder decreases, and therefore knocking can be prevented. Therefore, fuel efficiency can be improved by increasing the compression ratio of an air-fuel mixture.
United States Patent Application Publication No. US 2002/0072281 A1 (hereinafter, referred to as “US 2002/0072281”) discloses an engine for use in an outboard motor that supplies liquid fuel directly to a combustion chamber. In the engine of US 2002/0072281, a fuel injection valve used to inject fuel is disposed near an intake port. Two intake ports are provided for each cylinder, and the fuel injection valve is disposed between the two intake ports.
Japanese Patent Application Publication No. 8-200074 (hereinafter, referred to as “JP 8-200074”) discloses an engine that supplies vaporized fuel to a combustion chamber. In the engine of JP 8-200074, a fuel injection valve used to inject fuel is disposed near an exhaust port. Liquid fuel jetted from the fuel injection valve is supplied to a fuel vaporizing chamber. The fuel vaporizing chamber is contiguous to the combustion chamber, and is in a high-temperature state resulting from combustion heat transmitted from the combustion chamber. Therefore, when liquid fuel is supplied to the fuel vaporizing chamber from the fuel injection valve, the fuel is vaporized in the fuel vaporizing chamber. The fuel vaporized in the fuel vaporizing chamber is then supplied to the combustion chamber from near the exhaust port.